Virginia's Declaration of Rights was drawn upon by Thomas Jefferson for the opening paragraphs of the Declaration of Independence. It was widely copied by the other colonies and became the basis of the Bill of Rights.
The ridges found in flexible metal duct create turbulence and will diminish airflow. NEVER use 4 dryer duct or any plastic duct on a range hood installation. The minimum size duct allowed on most range hoods is 3 Â¼ by 10 which is equivalent to 6 round. More powerful hoods require even larger duct sizes up to 10 round. Consult the specifications for the model you have chosen.
Under-cabinet and wall mount kitchen exhaust hoods come in depths of approximately 20, 22, 24 and 27. 20 deep is fine for electric and low output gas tops, especially if they are only 18 - 20 above the range top. I suggest 22 or 24 deep when the hood is over 500cfm, and 27 deep if over 850cfm.